xen: debug: gcov: add condition coverage support

xen: debug: gcov: add condition coverage support

Condition coverage, also known as MC/DC (modified condition/decision
coverage) is a coverage metric that tracks separate outcomes in
boolean expressions.

This patch adds CONFIG_CONDITION_COVERAGE option to enable MC/DC for
GCC. Clang is not supported right now because Xen can't emit version
10 of LLVM profile data, where MC/DC support was added.

Also, use the opportunity to convert COV_FLAGS to cov-cflags-y, which
reduces amount of ifeqs in Rules.mk. Otherwise this patch had to add
another nesting level with "ifeq ($(CONFIG_CONDITION_COVERAGE),y)".

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>